Glass-ceramics having microcrystals of mica dispersed in the vitreous matrix have excellent dielectric properties, high resistance to thermal shock, and good machinability, and are considered to be promising materials capable of extending the range of use of high performance ceramics. Among others, glass-ceramics having microcrystals of fluorophlogopite dispersed therein are especially preferred materials because they additionally have excellent high-temperature stability.
As disclosed in Japanese Patent Publication No. 34775/'79 and U.S. Pat. No. 3,689,293 with the declaratio of priority, there is a known method of making such glass-ceramics which comprises preparing a powder mixture of starting materials in such proportions as to give a final product having a chemical composition capable of forming and containing a required amount of fluorophlogopite microcrystals, heating the powder mixture at a high temperature of at least about 1400.degree. C. to melt it and thereby form a vitreous matrix, cooling and solidifying the melt while forming it into a shape desired for the final product, and again heat-treating the shaped body at a high temperature of 750.degree. to 1100.degree. C. for a long period of time to obtain the desired product. In this method, the powder mixture must be heated at a high temperature of at least about 1400.degree. C. in order to melt it and form a homogeneous amorphous matrix. However, when the powder mixture containing a high proportion of fluorine is heated at such a high temperature, its reactivity is increased to cause considerable damage to the vessel used for the heating. Moreover, when a shaped body of large size is formed by pouring the melt into a desired mold and solidifying it, the temperature difference between the surface region and inner part of the shaped body being cooled is unavoidably increased as the temperature of the melt becomes higher. Thus, the solidified material is not uniform in structure between the surface region and inner part thereof and, in turn, the final product obtained after heat treatment is also inhomogeneous. Accordingly, it has been difficult to make large-sized products of good quality. In addition, this method has the disadvantage of involving a much greater heat energy cost because it includes the steps of heating the starting materials at about 1400.degree. C. to melt them, cooling and solidifying the melt, and again heat-treating the shaped body at 750.degree.-1100.degree. C.
Furthermore, this method involves the addition of B.sub.2 O.sub.3 for the purpose of reducing the softening point required for vitrification and promoting the growth of fluorophlogopite crystals. However, the inherently low melting point of B.sub.2 O.sub.3 is disadvantageous in that the resulting product has a reduced flexural strength and a heat resistance of as low as 1000.degree. C. or below.
According to another known method, glass-ceramics are made by mixing finely powdered fluorophlogopite crystals with a binder having a lower melting point (such as glass, phosphates, low-melting mica, etc.) and sintering this mixture. This method can cut down the great heat energy cost which constitutes one disadvantage of the foregoing method. However, the heat resistance is reduced because of its dependence on the melting point of the binder used for the formation of a matrix and the content of fluorophlogopite cannot be increased satisfactorily, so that the quality of the resulting product is deteriorated.
In order to overcome these disadvantages, the present inventors developed a new method of making glass-ceramics containing fluorophlogopite microcrystals which comprises mixing alkoxide compounds of silicon, aluminum, magnesium, potassium and boron with a fluorine compound, adding water to the mixture so as to effect hydrolysis of the aforesaid compounds, dehydrating and drying the resulting gel, and heat-treating the resulting dry solid at a temperature lower than its softening point (Japanese Patent Laid-Open No. 215548/'85). However, since the products made by this method still contain B.sub.2 O.sub.3, their flexural strength has an upper limit of 1200 kgf/cm.sup.2 and their heat resistance is as low as about 1000.degree. C.
As the range of use of glass-ceramic products is extended in recent years, there is a growing demand for glass-ceramic products having greater flexural strength and higher heat resistance while retaining their excellent machinability.